The present invention relates to water-soluble polymer powders which have been stabilized, i.e. protected against autoignition and oxidative degradation, and which are based on polyoxyalkylene glycol carboxylates, and also to a process for their preparation.
Water-soluble polymers based on oxyalkylene glycols and carboxylic acid derivatives have very recently obtained a large number of applications. Besides their use as dispersion stabilizers in the preparation of water-soluble copolymers (WO 97/30094) they are also used, inter alia, as a stabilizing agent (protective colloid) in the preparation of caking-resistant dispersion powders.
However, they are preferably used, and used in large quantities, as a flow agent for hydraulic binders, such as cement, lime, and also calcium-sulfate-bonded construction materials (EP-A 838 444), as pigment dispersants in the coatings and colorants sector, as processing aids for clays and porcelain slips in the sectors of ceramics and refractories (DE-A 43 00 239) and also in petroleum production. The structural variety of products of this type extends from copolymers with a random, alternating, block or comb-like structure made from a structural unit containing oxyalkylene glycol and from a carboxylic acid monomer (such as acrylic acid, methacrylic acid, maleic acid, (maleic anhydride), itaconic acid, fumaric acid, etc.) to graft copolymers of the abovementioned carboxylic acid monomers on polyoxyalkylene glycols or, respectively, derivatives of these (ethers, esters). These polanders, referred to below as polyoxyalkylene glycol carboxylates, may either be in the free acid form or in their salt form.
The advantage of copolymers of this time, for example in the sector for flow agent applications in the construction industry, is firstly the opportunity to use very small amounts to achieve long lasting handling qualities in cementaceous mixtures of construction materials (ready-mix concrete industry).
Secondly, the water requirement of these mixtures can be reduced so markedly that high-strength concretes can be produced (prefabricated). The products themselves are free from hazardous constituents, such as formaldehyde, and in this they differ from conventional cement flow agents, e.g. of EP-A 214 412 or DE-C 16 71 017. For numerous applications it is useful and desirable to prepare aqueous preparations of the abovementioned water-soluble polymers. However, if the polymers are to be used as an additive in dry mixes prepared in advance in the factory, the products have to be in a form which is solid and as finely dispersible as possible, i.e. in powder form.
Particular technical advantages of a powder, alongside logistical and economic advantages of powders over solutions, are improved storage stability and the absence of susceptibility to frost.
However, pulverulent polymers based on polyoxyalkylene glycol carboxylates tend to generate heat spontaneously, or even to autoignite, in particular if the powder has a very high specific surface area because it has been very finely ground, or if the amounts stored are large, for example in a silo. This tendency produces a major safety risk. Added to this is the fact that the relatively high temperatures occurring, and also the presence of atmosoheric oxygen, reduce the effectiveness of the products as the oxyalkylene glycol structural units undergo thermooxidative degradation processes.
The stabilizers or antioxidants known from the publications U.S. Pat. No. 4,070,304, U.S. Pat. No. 4,444,676, DE-A 2 557 619 and European Patent 38 876 comprise sterically hindered phenols, bisphenols, dialkyldiphenylamines, phenothiazines and substituted phosphites or mixtures of these classes of substance.
The effectiveness of the stabilizers firstly depends on the class of substance to which the particular representatives belong, and secondly is markedly affected by the compatibility with the substrate to be stabilized.
Many of the known stabilizers from the abovementioned groups are water-insoluble and are therefore highly suitable for stabilizing hydrophobic substrates, such as dispersion powders based on vinyl acetate, vinyl versetate, ethylene and 2-ethylhexyl acrylate (EP-A 751 175). As described in DE-A 42 26 288 and DE-A 43 00 892, the abovementioned water-insoluble stabilizers also have excellent comparability with pure substrates from the polyalkylene glycols group. They are therefore used successfully for thermooxidative stabilization of this class of substances.
However, problems occur if the extremely hydrophilic properties of the substrate to be stabilized make it incompatible with hydrophobic stabilizers, as is the case with the polyoxyalkylene glycol carboxylates described.
It was therefore an object of the present invention to develop water-soluble polymer powders which are based on polyoxyalkylene glycol carboxylates and which have been stabilized using conventional stabilizers to prevent spontaneous heating and thermooxidative decomposition.
According to the invention, this object was achieved by preparing stabilized polyoxyalkylene glycol carboxylate powders which comprise a stabilizer selected from the group consisting or phenols, amines, phosphites, thioethers, and also thioacids, in an amount adequate for stabilization, in particular from 0.01 to 10% by weight, based on the weight of the polymer powder. The stabilized powders are obtainable by adding the stabilizer in liquid or dissolved form to the aqueous polymer solution prior to its conversion into the powder form. Surprisingly, it has been found here that even hydrophobic stabilizers can be used to give excellent stabilization of the hydrophilic polyoxyalkylene glycol carboxylates, so that when a specimen of the polymer powder is heated at 140xc2x0 C. for 24 h in air the temperature in the interior of the specimen does not exceed 200xc2x0 C.
The water-soluble polymer powders which are to be stabilized and which are based on polyoxyalkylene glycol carboxylates are products which contain oxyalkylene glycol groups and carboxyl groups in their main and/or side chain. These polymers may have structures made from monomers based on unsaturated carboxylic acid (derivatives) and oxyalkylene glycol alkenyl ethers.
The polymers may also contain an unsaturated carboxylic acid as main chain and polyalkylene oxide side chains bonded via ester groups. Particularly suitable unsaturated carboxylic acids and, respectively, carboxylic acid derivatives are acrylic acid, methacrylic acid, maleic anhydride, maleic acid, fumaric acid, itaconic acid, and also itaconic anhydride. These polymers may be linear, short-chain branched, long-chain branched or else slightly crosslinked, and may be comb-shaped, star-shaped or dumbbell-shaped, or have other morphologies.
For the purposes of the present invention, the polymer powder to be stabilized may certainly also contain singly and/or multiply unsaturated vinyl- and/or acrylate-based monomers which are needed for the structure of the basic skeleton or on the side chains.
Examples of these are styrene, xcex1-methylstyrene, vinyl acetate, isobutene, diisobutene, cyclopentadiene, ethylene, propylene, methyl acrylate, methyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate, N-vinylpyrrolidone, acrylamide, methacrylamide, acrylamidomethylpropanesulfonic acid, styrenesulfonic acid, methyl vinyl ether, ethyl vinyl ether, allylsulfonic acid, butadiene and acrilonitrile.
Examples of the polymers stabilized according to the invention are polymers with a comb-type structure made from a methacrylic acid main chair and polyalkylene oxide side chains bonded via ester groups, maleic anhydride-styrene copolymers (partly) esterified with methylpolyethylene glycol, allylpolyethylene glycol-maleic acid copolymers, vinylpolyethylene glycol-maleic monoester copolymers, graft copolymers composed of a polyethylene glycol or, respectively, polypropylene glycol skeleton and maleic anhydride or, respectively, acrylic acid side chains, which themselves may in turn have been esterified or partly esterified, and also block copolymers made from methacrylic acid and ethylene glycol.
For stabilization to prevent autoignition, and also to prevent thermooxidative degradation of the polymers in powder form, use may be made of compounds based on phenols, and these can be divided into the following classes of compounds:
1. Alkylated monophenols having one or more alkyl radicals of from 1 to 18 carbon atoms, e.g. 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butylphenol, 2-tert-butyl-4,6-dimethylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,6-di-tert-butyl-4-n-butylphenol, 2,6-di-tert-butyl-4-i-butylphenol, 2,6-dicyclopentyl-4-methylphenol, 2-(xcex2-methyl-cyclohexyl)-4,6-dimethylphenol, 2,6-dioctadecyl-4-methylphenol, 2,4,6-tricyclohexylphenol, 2,6-di-tert-butyl-4-methoxymethylphenol, o-tert-butyl-phenol.
2. Alkylated hydroquinones having one or more alkyl radicals of from 1 to 18 carbon atoms, e.g. 2,6-di-tert-butyl-4-methoxyphenol, 2,5-di-tert-butylhydroquinone, 2,5-di-tert-amylhydroquinone, 2,6-diphenyl-4-octadecyloxyphenol.
3. Alkylidenebisphenols having an unsubstituted or substituted C1-C4-alkylidene radical, and/or sterically hindered polynuclear phenols, e.g. 2,2xe2x80x2-methylenebis(6-tert-butyl-4-methylphenol), 2,2xe2x80x2-methylenebis(6-tert-butyl-4-ethylphenol), 2,2xe2x80x2-methylenebis(4-methyl-6-(xcex1-methyl-cyclohexyl)phenol), 2,2xe2x80x2-methylenebis(4-methyl-6-cyclohexylphenol), 2,2xe2x80x2-methylenebis(6-nonyl-4-methylphenol), 2,2xe2x80x2-methylenebis(4,6-di-tert-butylphenol), 2,2xe2x80x2-ethylidenebis(4,6-di-tert-butylphenol), 2,2xe2x80x2-ethylidenebis(6-tert-butyl-4- or -5-isobutylphenol), 2,2xe2x80x2-methylenebis(6-xcex1-methylbenzyl)-4-nonylphenol), 2,2xe2x80x2-methylenebis(6-(xcex1,xcex1-dimethylbenzyl)-4-nonylphenol), 4,4xe2x80x2-methylenebis(2,6-di-tert-butylphenol), 4,4xe2x80x2-methylenebis(6-tert-butyl-2-methylphenol), 1,1-bis(5-tert-butyl-4-hydroxy-2-methylphenyl)-butane, 2,6-di(3-tert-butyl-5-methyl-2-hyvdroxybenzyl)-4-methylphenol, 1,1,3-tris(5-tert-butyl-4-hydroxy-2-methylphenyl) 3-n-dodecylmercaptobutane, ethylene glycol-bis[3,3-bis(3xe2x80x2-tert-butyl-4xe2x80x2-hydroxyphenyl)butyrate], 3,8-bis(3-tert-butyl-4-hydroxy-5-methylphenyl)di-cyclopentadiene, bis[2-(3xe2x80x2-tert-butyl-2xe2x80x2-hydroxy-5xe2x80x2-methylbenzyl)-6-tert-butyl-4-methylphenyl] terephthalate, 2,2xe2x80x2-isobutylidenebis(4,6-dimethylphenol), 4,4xe2x80x2-butylidenebis(2-tert-butyl-5-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane.
4. Phenolic benzyl compounds, e.g. 1,3,5-tri(3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene, bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide, isooctyl 3,5,-di-tert-butyl-4-hydroxybenzylmercapto acetate, bis(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)dithioterephthalate, tris(3,5-di-tert-butyl-4-hydroxybenzyl)1,3,5-isocyanurate, tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl) 1,3,5-isocyanurate, dioctadecyl 3,5-di-tert-butyl-4-hydroxybenzylphosphonate, the calcium salt of monoethyl 3,5-di-tert-butyl-4-hydroxybenzylphosphonate.
5. Phenol/diphenyl thioether compounds, e.g. 2,2xe2x80x2-thiobis(6-tert-butyl-4-methylphenol), 2,2xe2x80x2-thiobis(4-octylphenol), 4,4xe2x80x2-thiobis(6-tert-butyl-3-methylphenol), 4,4xe2x80x2-thiobis(6-tert-butyl-2-methylphenol).
6. Acylaminophenols, e.g. 4-hydroxylauranilide, 4-hydroxystearanilide, 2,4-biscylmercapto-6-(3,5-di-tert-butyl-4-hydroxyanilino)-s-triazine, octyl N-(3,5-di-tert-butyl-4-hydroxyphenyl)-carbamate.
7. Phenolic esters, such as esters of xcex2-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid or of xcex2-(5-tert-butyl-4-hydroxy-3-methylphenyl)propionic acid with mono- or polyhydric alcohols, e.g. methanol, diethylene glycol, octadecanol, triethylene glycol, 1,6-hexanediol, pentaerythritol, neopentyl glycol, trishydroxyethyl isocyanurate, thiodiethylene glycol, dihydroxyethyloxalamide.
8. Phenolic amides, such as amides of xcex2-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid, e.g. N,Nxe2x80x2-bis(3,5-di-tert-butyl-4-hydroxyphenylpropionyl) -hexamethylenediamine, N,N-bis(3,5-di-tert-butyl-4-hydroxyphenylpropionyl)trimethylenediamine, N,Nxe2x80x2-bis(3,5-di-tert-butyl-4-hydroxyphenyl-propionyl)hydrazine.
Particular amine-based stabilizers used are secondary or tertiary amines having aliphatic, araliphatic, aromatic or cyclic radicals or N-heterocycles (e.g. unsubstituted or substituted phenothiazines, benzothiazines or quinolines). Examples of these are:
N,Nxe2x80x2-diisopropyl-p-phenylenediamine, N,Nxe2x80x2-di-sec-butyl-p-phenylenediamine, N,Nxe2x80x2-bis(1,4-dimethylpentyl)-p-phenylenediamine, N,Nxe2x80x2-bis(1-ethyl-3-methylpentyl)-p-phenylenediamine, N,Nxe2x80x2-bis(1-methylheptyl)-p-phenylenediamine, N,Nxe2x80x2-dicyclohexyl-p-phenylenediamine, N,Nxe2x80x2-diphenyl-p-phenylenediamine, N,Nxe2x80x2-di(naphth-2-yl)-p-phenylenediamine, N-isopropyl-Nxe2x80x2-phenyl-p-phenylenediam, N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine, N-(1-methylheptyl)-Nxe2x80x2-phenyl-xcex2-phenylenediamine, N-cyclohexyl-Nxe2x80x2-phenyl-p-phenylenediamine, 4-(p-toluenesulfonamido)diphenylamine, N,Nxe2x80x2-dimethyl-N,Nxe2x80x2-di-sec-butyl-p-phenylenediamine, diphenylamine, N-allyldiphenylamine, 4-isopropoxydiphenylamine, N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, octylated diphenylamine, e.g. p,pxe2x80x2-di-tert-octyldiphenylamine, 4-n-butylaminophenol, 4-butyrylaminophenol, 4-nonanoylamincphenol, 4-dodecanoylaminophenol, 4-octadecanoylaminophenol, di(4-methoxyphenyl)amine, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, 2,4-diaminodiphenylmethane, 4,4xe2x80x2-diaminodiphenylmethane, N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-4,4xe2x80x2-diaminodiphenylmethane, 1,2-di(phenylamino)ethane, 1,2-di[2-(methylphenyl)amino]ethane, 1,3-di(phenylamino)propane, (o-tolyl)biguanide, di[4-(1xe2x80x2,3xe2x80x2-dimethylbutyl)phenyl]amine, tert-octylated N-phenyl-1-naphthylamine, mixtures made from mono- and dialkylated tert-butyl-/tert-octyldiphenylamines, 2,3-dihydro-3,3-dimethyl-4H-1,4-benzothiazine, phenothiazine, n-allylphenothiazine, styrenated diphenylamine, polymerized 2,2,4-trimethyl-1,2-dihydroquinoline.
Among the phosphites, stabilizers which may be used (esters of phosphorus acid) are compounds having aliphatic, araliphatic and/or aromatic radicals. Examples of these are: trimethyl phosphite, triethyl phosphite, triisobutyl phosphite, triphenyl phosphite and tris[4-nonylphenyl] phosphite.
Thioethers which have proven particularly suitable are esters of thiodiacetic acid or thiodipropionic acid, e.g. dilauryl 3,3xe2x80x2-thiodipropionate or distearyl 3,3xe2x80x2-thiodipropionate.
Among the thioacids, use is preferably made of dithiocarbamic and/or dithiophosohoric acid or salts thereof.
The stabilizers may be used either individually or in combination. Particular preference is given to the use of sterically hindered phenols and amines, and also phenothiazine.
The amounts added according to the invention depend on the: effectiveness of the stabilizer and are generally from 0.01 to 10% by weight, in particular from 0.1 to 1% by weight, based on the polymer powder, the particle size of which is preferably from 10 to 100 xcexcm. In one preferred embodiment, the novel mixtures made from water-soluble polymer powder and stabilizer also comprise up to 5% by weight of a water-soluble aminosulfonic acid compound, such as amidosulphonic acid and/or taurine, in the form of the free acid or as a salt.
An important factor for the invention is that the stabilizer in liquid or dissolved form is added to the aqueous polymer solution prior to its conversion into the powder form, since only then does the thermooxidative stabilizing action become effective.
To emulsify the stabilizer in the aqueous polymer solution, the stabilizer is added, either in liquid or in dissolved form (for example in a suitable organic solvent) to the appropriate aqueous polymer solution, whereupon a microemulsion forms, made from stabilizer and polymer compound, and the size of the finely divided oil droplets of the stabilizer is  less than 100 nm.
Surprisingly, the formation of this microemulsion is successful even without other additives in the form of surfactants or protective colloids, and also without the use of high-shear dispersion procedures.
In order to convert the microemusion composed of stabilizer and polymer compound into the powder form it is necessary to remove the liquid phase composed of water and of any organic solvent used, and this may readily be carried out by the usual methods, e.g. spray drying.
It has proved particularly advantageous here for the pH of the aqueous polymer solution prior to conversion into the powder form to be set to 6-10, preferably 7-9. The, solids content of the aqueous polymer solution here is preferably from 10 to 50% by weigh. With the aid of this emulsification step it is even possible to use hydrophobic compounds to stabilize the water-soluble polymer powders based on polyoxyalkylene glycol carboxylates, in which case polymer powder and stabilizer are converted by way of adsorptive interactions into an extremely finely divided colloidal system. The manner in which the colloid particles with their polyelectrolyte molecules are stabilized here, electrostatically (carboxyl groups) and also sterically (polyoxyalkenyl glycol groups), achieves a thermo-dynamically preferred state.
The large surface area resulting from the fine division of the stabilizer also means that every molecule of the substrate to be stabilized is reached, and this gives exceptional efficacy.
The polymer powders protected according to the invention against autoignition and oxidative degradation therefore have unexpectedly high thermooxidative stability, even when exposed to high temperatures and oxidizing action (air, oxygen).